The present invention relates to an X-ray apparatus with focus position control formed on the rotary anode of an X-ray tube with magnetic bearing. The invention is applicable in general terms to X-ray diagnosis and particularly in X-ray scanning.
In X-ray diagnosis, the X-ray used is generally produced from an X-ray tube carrying a generally rotary anode, on which the impact of an electron beam defines a focus, which constitutes the X-ray source.
The X-ray tube is installed in a protective sheath (against high voltage and X-ray), with which it constitutes an X-ray emitter means. When the latter is mounted on an X-ray apparatus, it is only the actual sheath which is fixed thereto on supports of reference surfaces provided for this purpose. If it is wished to easily ensure the rigidity of this fixture, relative movements between the focus of the X-ray tube and the mechanical references of the sheath containing the same are difficult to control.
These movements mainly have two origins. The first origin is of a mechanical nature. The fixing parts connecting the X-ray tube in its sheath are necessarily formed from electrically insulating material. Certain of these materials are not rigid and are not in fact mechanical materials, said insulants having an excessive flexibility and compressibility. The second origin is due to thermal movements. During the normal use of an X-ray assembly, the temperature differences to which it is exposed are considerable, particularly for certain elements of the X-ray tube such as the anode disk, the different spindles and the rotor to which the anode is coupled in rotation.
Thus, between a cold start and balanced temperature operation, by no means negligible expansions (0.5 mm) occur, which lead to the displacement of the focus with respect to an initial position thereof at cold start. This displacement of the focus due to the thermal movement mainly takes place in an axis parallel to that along which the anode disk is connected to the rotor.
The inevitable thermal movements of the focus are prejudicial to the operation of X-ray equipment particularly in X-ray scanning where the image quality is limited by acquisition defects with respect to the analog signals, particularly due to deviations relative to the spatial position of the focus.